Metrological scale

ABSTRACT

A method of applying a marking onto a metrological scale. The method includes locating one or more markings on the scale substrate in a provisional state; checking whether the one or more markings located on the scale substrate are acceptable; and finalising the one or markings which are acceptable so as to transform the one or more markings into a finalised state.

This is a Divisional of application Ser. No. 12/936,996 filed Oct. 8,2010, which is a National Phase entry of International Application No.PCT/GB2009/001014 filed Apr. 21, 2009, which claims the benefit ofBritish Application No. 0807242.3 filed Apr. 21, 2008. The disclosure ofthe prior applications is hereby incorporated by reference herein in itsentirety.

The present invention relates to metrological scale, a method ofmanufacturing said scale and an apparatus for making markings onmetrological scale. In particular, the invention relates to ametrological scale comprising markings and a method of applying markingsto said scale.

A known form of scale reading apparatus for measuring relativedisplacement of two members comprises a scale on one of the membershaving scale marks defining a pattern and a readhead provided on theother member. An optical scale reading apparatus has a light source forilluminating the scale, a detector in the readhead responsive toresultant light patterns to produce a measure of relative displacementof the scale and readhead. A scale having its marks in a periodicpattern is known as an incremental scale and the readhead provides anoutput of up and down counts. For instance, such a scale is described inPublished European Patent Application no. 0207121. A scale may beprovided with reference marks which when detected by the readhead enablean absolute or unique position of the readhead to be determined. Forexample, such a scale is disclosed in Published International PatentApplication WO 2005/124282. Mass produced scale may have an incrementaltrack and a plurality of reference marks. The user may wish to use someof these reference marks but not others. A reference mark could beembedded within the incremental track, or it could be placed adjacent tothe incremental track, for instance in a separate track.

Absolute scales are also known which typically comprise featuresdefining a series of absolute codes. During use a readhead reads codesas it moves along the scale and translates the codes into absoluteposition information. Such a scale is disclosed in International PatentApplication no. PCT/GB2002/001629. It can be useful in somecircumstances to apply limit marks adjacent or on the scale so as todefine the limit of relative movement of the readhead and scale.

In many machines, for example machine tools, the end points of movementof the machine part are defined by limit switches. These for example maybe mechanical, magnetic or optical.

The present invention relates to improvements in the methods andapparatus for applying markings onto scale substrates.

According to a first aspect of the invention there is provided a methodof applying a marking onto a metrological scale, comprising: locatingone or more markings with respect to the scale substrate in aprovisional state; checking whether the one or more markings locatedwith respect to the scale substrate are acceptable; and finalising theone or more markings which are acceptable.

This method allows the marking to be checked before it is finalised.This is particularly advantageous if markings are being applied to thescale substrate in situ or added to material already processed to form ascale (e.g. incremental or absolute).

Finalising can comprise transforming the one or more markings into afinalised state. In other words the method can comprise subjecting themarking to a finalisation process in which the marking undergoes atransformation that makes the marking permanent, i.e. less readilyremovable from the scale than in its provisional state.

The form of the marking could change, e.g. transform, between itsprovisional and finalised state. For instance, the marking in itsprovisional state could be a liquid placed on the scale which can beactivated during finalisation to cause it to react with the scalethereby permanently marking the scale. For instance, the liquid could bea selectively activatable etchant which when activated reacts with thescale substrate to change locally the form of the scale's substrate. Forinstance, the etchant could cause a smooth surface to become rough orvice versa. The etchant could erode a local area of the scale substratein order to form a cavity.

Preferably, the form of the marking in its provisional and finalisedstates is substantially the same. The form can comprise the appearanceof the marking, for instance the shape and size of the marking. Inparticular, preferably the form comprises the properties of the markingwhich are used for detection of the marking by a readhead. Accordingly,preferably finalising does not substantially alter the form of themarking. This can be advantageous as the checking step can then give agood indication of the quality of the finalised marking before it isactually finalised.

Finalising can comprise fixing the marking to the scale substrate sothat it is less readily removable. This can comprise interacting withthe marking to fix it to the scale. As explained in more detail below,fixing can comprise transferring the marking onto the scale. This couldinclude causing the marking to bond with the scale.

Finalising can comprise subjecting the marking to at least one ofpressure, a heat source, an electromagnetic radiation (“EMR”) source anda chemical source. For instance, a pressure tool, such as a roller,could be used to apply pressure to the marking. Optionally, the pressuretool could comprise an impact apparatus. The impact apparatus couldcomprise an impact element having an end for placement against themarking. The impact end's profile can be substantially the same as themarking's profile. The impact apparatus could further comprise animpactor for applying an impact force to the impact element. The amountof pressure needed in order to finalise the marking will depend on manyfactors including, for example, the type of pressure sensitive adhesiveused on the marking. Suitable pressures could be at least 5 N per 10mm², preferably at least 10 N per 10 mm², more preferably at least 15 Nper 10 mm², and preferably not more than 100 N per 10 mm², for examplenot more than 50 N per 10 mm² For instance, a suitable pressure could beapproximately 20 N per 10 mm².

A heat source could be used to subject the marking to temperatures of atleast 50° C., preferably at least 70° C., for instance at least 100° C.It can be preferable that the marking is not subjected to temperaturesgreater than 300° C., more preferably not greater than 200° C.

An EMR source could be used to expose the marking to EMR, for instancean optical source could be used to expose the marking to optical EMR.Optical EMR includes EMR in the range of infra-red to ultra-violet(“UV”). In particular, an Ultra-Violet (“UV”) source could be used toexpose the marking to UV EMR.

Finalising can comprise curing the marking. Suitable curing techniquesinclude exposing the one or more markings to an EMR source. Suitable EMRsources include an optical source, such as a visible light source or aUV source. Other suitable curing techniques comprise: heat curing, e.g.exposing the one or more markings to a heat source; and chemical curing,e.g. exposing the one or more markings to a chemical which cures themarking.

Locating can comprise applying a marking onto the scale substrate usinga reversible process. For instance, the marking in its provisional statemight be a liquid applied to the scale substrate. Accordingly, applyingcan comprise applying a marking in a liquid state onto the scalesubstrate. The liquid could be a photo-curable liquid, in particular itcould be a UV-curable liquid. The liquid can comprise an ink, forinstance a photo-curable ink, or more particularly a UV-curable ink. Inembodiments in which the marking in its provisional state is a liquid,finalising can comprise transforming a marking into a solid state. Forexample, finalising can comprise curing the one or more liquid markings.

The step of applying one or more markings to the scale substrate mayinclude the step of pinning the marking. Pinning can help keep themarking in place without finalising it. This is especially useful whenthe marking in its provisional state is a liquid as it can prevent theliquid from flowing freely around the scale substrate. When the markingis in a liquid form in its provisional state then pinning can compriseforming a skin over the exposed surface of the marking so as to hold theform and position of the marking.

Applying can comprise applying a marking in a solid state. The markingcan comprise an adhesive for securing the marking to the scale. In thiscase, finalising can comprise transforming the adhesive. Transformingthe adhesive can comprise curing the adhesive.

Locating can comprise positioning a marking carrier adjacent the scaleand finalising can comprise transferring the marking from the markingcarrier onto the scale. For instance, the marking could comprise a pieceof material having an adhesive on at least one side, e.g. a sticker.Optionally, the marking could be a decal which can be transferred ontothe scale substrate. Accordingly, in these cases the marking istransformed by it being transferred from the marking carrier onto thescale substrate. Finalising could comprise applying pressure to the oneor more markings so that the marking becomes bonded, and hencetransferred, to the scale substrate. Optionally, a thermal transfercould be used to cause localised melting and bonding of the markingcarrier to the scale substrate. Accordingly, when the marking carrier issubsequently removed the localised area that has been subjected to thethermal transfer process remains on the scale substrate.

Preferably, when placed on the metrological scale, the marking carrierdoes not adversely affect the readhead's detection of the metrologicalscale's position features.

The marking carrier could comprise a layer of material to which themarking is removably attached. For instance, the marking carrier couldbe sheet-like. Preferably, the marking is carried on the underside ofthe marking carrier. Preferably, the layer of material permits thedetection of the metrological scale's position features therethrough.

Preferably, the surface area of the underside of the marking carrier islarger than that of the marking. Preferably, the surface area of theunderside of the marking carrier is at least twice the size as that ofthe marking, more preferably at least three times the size, for exampleat least four times the size. The underside of the marking carrier couldcomprise an adhesive for temporally securing the marking carrier to thescale during checking and/or finalising. Preferably, any adhesive on themarking carrier is different to any adhesive on the marking. Preferably,any adhesive on the marking carrier is weaker than any adhesive on themarking. In particular, preferably the marking carrier's adhesive is notcurable by the method used to finalise the marking.

The marking carrier can comprise at least one removable backing layerwhich covers at least the marking's scale contact surface. Optionally,the at least one backing layer covers the marking carrier's scalecontact surface, i.e. its underside. The same or separate backing layerscould be used to cover the marking carrier and marking. In embodimentsin which the marking comprises an adhesive, the backing layer isremovable so as to expose the marking's adhesive. Preferably, inembodiments in which the marking carrier comprises an adhesive, thebacking layer is removable so as to expose the marking carrier'sadhesive.

Preferably, the marking carrier has greater rigidity than the markingmaterial. Preferably, the marking carrier is substantially thicker thanthe marking material. Preferably, the marking carrier is at least twotimes thicker than the marking material, more preferably at least 10times thicker. Preferably, the marking material is not more than 10 μmat its edges, more preferably not more than 5 μm at its edges,especially preferably not more than 2 μm at its edges, for example 1 μmat its edges. The marking material can be thinner at its edges thantoward its middle. Optionally, the marking material has a substantiallyuniform thickness. Preferably, the marking carrier is at least 10 μm inthickness, more preferably at least 50 μm, especially preferably atleast 100 μm, for example at least 200 μm.

Checking can comprise checking the position and/or orientation of themarking. Checking can comprise checking other characteristics of themarking. For instance, checking can comprise checking the size and/orclarity of the marking. It could also comprise, for instance, checkingthat the marking is complete and/or of the correct type or form.

Checking can comprise a detector detecting the marking and a processordevice analysing the detected marking to determine if it complies withpredetermined criteria. Checking whether the one or more markings areacceptable may be a visual check. It may be carried out manually by anoperator. Checking may be automated. For instance, in embodiments inwhich the marking is a visual marking, the marking may be checked e.g.using a vision machine.

The method can further comprise removing any markings which are notacceptable.

Preferably, the metrological scale comprises a series of positionfeatures defining an incremental or absolute scale. Preferably, the oneor more markings define a reference position on the incremental orabsolute scale. The reference position can, for instance, be a boundaryof relative movement between the scale and a readhead reading the scale.In this case the marking can be a limit mark. The reference position canbe a unique reference point along the length of the scale. For instance,in the case of an incremental scale, the marking could comprise areference mark which defines a reference point, for instance an absolutereference point, along the length of the incremental scale.

The metrological scale can be a magnetic or an inductive metrologicalscale. The metrological scale can be a capacitive metrological scale.Optionally, the metrological scale is an optical metrological scale. Inthis case, the metrological scale could be transmissive in which areadhead reads the scale via detecting light transmitted through thescale. Optionally, the metrological scale could be reflective in which areadhead reads the scale via detecting light reflected off the scale.

As will be understood, there are many suitable ways in which theposition features can be defined on a scale. For instance, positionfeatures can be defined by features having particular electromagneticradiation (EMR) properties, for example particular optical properties,for instance by the particular optical transmissivity or reflectivity ofparts of the scale. Accordingly, a position feature could for example bedefined by parts of the scale having a minimum reflectivity ortransmissivity value. Optionally, a position feature could for examplebe defined by parts of the scale having a maximum reflectivity ortransmissivity value. In the case of a magnetic metrological scale,position features can be defined by features having particular magneticproperties or for instance by the presence or absence of ferromagneticmaterial. In the case of capacitive scale position features can bedefined by features having particular capacitive properties, and in thecase of an inductive scale, position features can be defined by featureshaving particular inductive properties.

The position features can take the form of lines, dots or otherconfigurations which can be read by a readhead. Preferred configurationsfor one-dimensional scales can comprise lines extending across theentire width of a track in a dimension perpendicular to the measuringdimension.

Likewise, the one or more markings can take the form of lines, dots orother configurations.

Preferably, the marking can be cleaned but not removed with anappropriate cleaner. For instance, the cleaner could be a liquid, forinstance a solvent. Preferably, the marking can be cleaned but notremoved with a first solvent and removed with a second solvent. Forinstance, a marking in the form of a sticker may be applied to thescale. This sticker could be cleaned with isopropyl alcohol (“IPA”)without damage and removed with, for example, acetone or xylene.Furthermore, the sticker could be finalised using, for example, heat.Optionally, a marking may be applied in a provisional state in the formof UV curable ink using a pen to draw the marking on the scale. Theprovisional marking may be cleaned with water and removed with IPA.Furthermore, the provisional marking could be finalized with UV light.

Accordingly, this application describes a method of producing markingson a scale substrate, comprising: applying one or more markings to thescale substrate using a reversible process, checking whether the one ormore markings are of acceptable quality; and finalising the one or moremarkings which are of acceptable quality.

According to a second aspect of the invention there is provided anapparatus for applying a marking to a scale substrate, comprising: ascale locator for positioning the apparatus in a defined positionrelative to the scale substrate in at least one dimension; an applicatorfor locating one or more markings on the scale substrate in aprovisional state; a detector for checking the markings; and a finaliserfor finalising the one or markings which are acceptable so as totransform the one or more markings into a finalised state.

The apparatus can further comprise a remover for removing from the scalesubstrate one or more markings which are not acceptable.

Another aspect of the present invention provides apparatus for applyinga marking to a scale substrate, the apparatus comprising: a scalelocator for positioning the apparatus in a defined position relative tothe scale substrate in one dimension; a reagent storage region; and anapplicator which provides a conduit for the reagent from the reagentstorage region to the scale substrate.

In a preferred embodiment, the applicator is configured to apply thereagent to the scale substrate in the desired shape of the marking.Preferably the desired shape of the marking is a line.

Preferably the applicator is movable between first and second positions.The first position may be in contact with the scale substrate when thescale substrate is engaged with the scale locator. The second positionmay be retracted.

The reagent storage region may be sized to contain sufficient reagentfor one or multiple markings.

The reagent storage region and the applicator may be provided in oneintegral unit.

The apparatus may further comprise a curing source, for example anoptical curing source.

According to a further aspect, the invention provides a kit of partscomprising: a scale comprising a series of position features; and anapparatus for applying a reference mark onto the scale, the apparatuscarrying marker matter which can be applied to a scale substrate in aprovisional state and subsequently transformable into a finalised stateon the scale substrate via a finalisation procedure.

The apparatus can comprise a storage region for the marker matter and anapplicator for transferring the marker matter from the storage regiononto a scale substrate. For example, the marker matter can be a liquidand the applicator can comprise at least one conduit for transferringthe marker liquid from the storage region onto a scale substrate.

The marker matter can comprise an object comprising an adhesive backingthat is transformable into a finalised state on the scale substrate viaa finalisation procedure.

The marker matter can be a decal and the apparatus can comprise a decalcarrier sheet. Preferably, the decal carrier sheet is configured suchthat it does not interfere with the detection of markings on a scalesubstrate on which the decal carrier is placed.

The apparatus can comprise features for aiding alignment of the markingto the scale substrate. The apparatus can comprise a scale locator forpositioning the apparatus in a defined position relative to the scalesubstrate in at least one dimension.

The kit can further comprise a tool for finalising the marking.

According to a yet further aspect of the invention there is provided anapparatus for applying a reference mark onto the scale, for use with akit of parts as described above.

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 is a side view of a scale and readhead;

FIG. 2 is a plan view of the scale and readhead of FIG. 1;

FIG. 3( a) is a flow diagram showing the outline of the method;

FIG. 3( b) is a schematic illustration of a first way of carrying outthe steps of the flow diagram shown in FIG. 3 a;

FIG. 3( c) is a schematic illustration of a second way of carrying outthe steps of the flow diagram shown in FIG. 3 a;

FIG. 3( d) is a schematic illustration of the finalisation step of FIG.3 a being carried out using impact means;

FIG. 4 is a schematic illustration of a system suitable forautomatically carrying out the method of 3 a;

FIGS. 5( a), (b) and (c) illustrate end, side and bottom viewsrespectively of a device for manually applying reference marks;

FIG. 6 illustrates a mask for applying the reference mark;

FIG. 7 illustrates a device for applying a reference mark by tampoprinting;

FIG. 8 illustrates an alternative blade for use in FIG. 5;

FIG. 9 illustrates an alternative embodiment of the device in FIG. 5;and

FIG. 10A-10C illustrate ink on a blade tip before scraping, a scraperand ink on a blade tip after scraping.

FIG. 1 illustrates a scale 10 mounted on a first machine part (notshown) and a readhead 12 mounted on a second machine part (not shown).The second machine part and thus the readhead are movable relative tothe scale in the direction shown by arrow A. FIG. 2 illustrates thescale 10 which has an incremental track 14 comprising a periodic patternof scale marks 16 made up of alternate reflecting and non-reflectinglines. Reference mark 18 is embedded in the incremental track 14. Onlyone reference mark is shown embedded in the incremental track but aswill be understood there can be a number of identical or uniquereference marks embedded in the incremental track 14. Alternatively oradditionally markings may be provided in a separate scale track, such asillustrated by reference mark 11. When there are several referencemarks, the end user may wish to select a particular marking and ignorethe other markings. Reference marks could be, for instance, used aslimit marks which indicate the boundaries of relative motion of thereadhead 12 and scale 10.

The readhead 12 is provided with standard incremental optics 2 forreading the incremental scale and reference mark and/or limit switchsensing optics 4 and 5 for reading the reference marks or limit marks.

Incremental scale may be manufactured in large quantities and it may bedesirable to provide different patterns of reference marks and limitswitches suitable for any particular application. Thus it may beadvantageous to apply the reference marks and limit switches in aseparate step from making the incremental scale.

FIG. 3( a) is a flow diagram outlining the method of applying scalemarkings to a scale substrate. This method may be automated or carriedout manually in situ; these alternatives will be described in moredetail.

In a first step 19 a marking applicator and the scale substrate arepositioned accordingly. A scale marking (or scale markings) is thenapplied to the scale substrate using a reversible process 20. In thedescribed embodiment, a reagent, such as photo-curable ink, for exampleUV curable ink, is used which can be removed, for example by wiping withsolvent and cloth.

In a third step, the applied marking is checked to determine whether itis of acceptable quality 22. This step may use criteria such as markingposition, shape, density etc. The quality of the marking may be checkedmanually, e.g. a visual inspection by an operator or may be checkedusing an automated process.

If the markings pass the quality test, then the markings are finalised26, for example by exposure to UV light, a particular chemical, heat orany other suitable finalising procedure. If the markings fail thequality test, then a removal process is used to remove at least thefailed marking or either a subset or all of the markings 24. Themarkings are then reapplied using the above process.

FIG. 3( b) schematically illustrates one embodiment of how each of thesesteps might be carried out. As shown, at the first step 19, anapplicator 100, such as that described below in connection with FIG. 9(or indeed any of the applicators described in connection with FIGS. 5to 10 c) is positioned relative to the scale substrate 10 in a desiredposition. The applicator 100 is then actuated by pressing down on it inthe direction of arrow B, so that the marking is applied to the scalesubstrate 10. In this case, contact between the applicator's blade 104and the scale substrate 10 causes a line of chemically curable ink, suchas an epoxy, for instance, an epoxy that is the reaction product ofepichlorohydrin and bisphenol-A, to be applied to the scale substrate10. The quality of the marking is then checked by passing a readhead 21over the reference mark. The output of the readhead 21 is displayed onan oscilloscope 23 (or for instance a computer display) so that the usercan check the reference mark. In particular the user will be looking tocheck the quality of the reference mark such as the size, uniformity,and position of the reference mark. If it appears to the user that thequality of the reference mark is unacceptable then at step 24 the usercan apply some solvent, for instance isopropyl alcohol (“IPA”) 25, ontothe marking and wipe the scale substrate clean using a swab 27, so as toremove the marking 18 completely leaving no evidence that it was ever onthe scale substrate 10. Otherwise, at step 26 the user can finalise themarking, which in this embodiment comprises spraying it with a chemical,for instance a curing catalyst such as, for example, aliphatic polyamineor a polyamide, which on contact with the marking cures it so as totransform the marking from a liquid state into a solid state that hasadhered to the scale substrate 10.

FIG. 3( c) schematically illustrates an alternative embodiment of howeach of these steps might be carried out. In this case the marking 18comprises a thermally transferrable decal made from polyester backedwith a heat curing adhesive, such as the epoxy reaction product ofepicholorohydrin and bisphenol-A with a heat-reactive cross-linkingagent such as phenol-formaldehyde, which is carried on a marking carrier17 made from polyester backed with a weak adhesive such as, for example,acrylic or polyvinyl acetate (PVA). As shown, the marking 18 is lightabsorbent and the marking carrier is transparent 17. A tab 13 whichlacks adhesive is provided for ease of manipulation of the markingcarrier 17. This decal marking 18 has a substantially uniform thicknessof 5 μm and the marking carrier has a substantially uniform thickness of100 μm. Although not shown in the figures, the marking carrier cancomprise alignment marks for aiding alignment of the decal marking 18and the scale substrate 10.

At the first step 19 the user peels the marking carrier 17 complete withdecal marking 18 from a backing paper (not shown) so as to expose theweak adhesive layer on the back of the marking carrier 17 and then, viathe tab 13, manipulates the marking 18 to a desired location over thescale substrate 10. At the second step 20, the user then brings themarking carrier 17 and decal marking 18 into contact with the scalesubstrate 10 so that the adhesive on the back of the marking carrier 17sticks to the scale substrate 10. The decal marking 18 is then held in aprovisional state on the scale substrate 10 and so its position can bechecked at step 22 by passing a readhead 21 over it and checking thereadhead's 21 output on an oscilloscope (not shown), or for instance acomputer display, in a similar manner to that described above inconnection with FIG. 3 b. As the marking carrier 17 is transparent tothe readhead 21 it has minimal effect on the readhead's 21 output andtherefore gives a true representation of how the decal marking 18 willappear on the scale once it has been finalised. If it appears to theuser that the quality of the reference mark is unacceptable then at step24 then user can peel the marking carrier 17 and decal marking 18 offthe scale substrate 10 so as to remove them completely, without leavingany deposits on the scale substrate and hence leaving no evidence thatthey were ever on the scale substrate. Otherwise, at step 26 the usercan finalise the marking. In this embodiment, this comprises heating thedecal marking 18 using a heat source 23 so as to cause adhesion of thedecal marking 18 onto the scale substrate 10. As shown, the decalmarking 18 can be heated by a directional heat source 23 which comprisesa fan and at least one electrically resistive element which when poweredgenerates heat. Accordingly, once the decal marking 18 has adhered tothe scale substrate 10, the marking carrier 17 can be peeled off thescale substrate, leaving the thin decal marking 18 permanently in placeon the scale substrate 10. Again, the marking carrier 17 leaves nodeposits on the scale substrate 10 and hence leaves no evidence that itwas ever on the scale substrate. This process enables a very thin decalmarking 18 to be applied to the scale substrate. The use of such a thindecal marking 18 helps to avoid the collection of debris at its edges,reduce the likelihood of peeling of the edges peeling, and helpsmaintain a shallow scale profile.

As will be understood, there are other ways of transferring decals ontoa substrate. For instance the decal marking 18 could be impacttransferrable. As illustrated in FIG. 3( d), in this case an impactelement 3 having an end of similar shape and dimension to the marking 18could be provided. At the finalising step 26, the impact element 3 couldbe placed on the marking 18 and then impacted via an impactor 7 so as tocause the decal to transfer onto the scale substrate by virtue of thepressure sensitive adhesive on the surface of the decal which faces thescale. Alternatively, the decal marking 18 could be transferrable onactivation by a liquid, for instance water. Accordingly, at thefinalising step decal marking 18 could be wetted causing it to adhere tothe scale substrate 10.

FIG. 4 is a schematic illustration of an automated process of applyingthe markings to a scale substrate. FIG. 4 is a continuous process butthis method is also suitable for discrete lengths of scale. Furthermore,the method steps could be separated.

The scale substrate 30 is passed into the scale marking apparatus 32.The scale substrate may be fed in the direction of arrow A by knownmeans, such as rollers 34, 36. Typically the scale substrate already hasscale markings, for instance incremental scale markings, formed on it.

Any additional marking, for instance a reference marking, is applied tothe scale substrate using, for instance, an ink jet printer 38. Acurable ink which requires a separate curing step for the finalisingstep is chosen, for example a photo-curable ink A suitable ink would bea UV curable ink. A photo-curable ink may require pinning which is apartial cure sufficient to prevent beading of the ink whilst stillallowing the ink to be removable. FIG. 4 shows a pinning step 40, whichuses a UV light source used to pin the applied scale markings. Theexposure time and intensity of this light source is controlled to ensureonly cure the exposed surface of the ink, thereby forming a skin. As theink has not fully or substantially cured the marking is still removable.

The scale substrate is then passed under a vision machine 42. Thiscomprises a camera, light source and processor (not shown). The lightsource illuminates the scale and the camera produces an image of themarking which is assessed by the processor. An algorithm is used todetermine whether the marking is of acceptable quality; the algorithmmay use parameters such as width, edge quality, orientation, position,continuity and contrast in determining the quality of the markings. Thevision system can be used to assess the cosmetic appearance of themarking in addition to other factors, such as its position.

If the markings fail the quality test, the substrate is taken to acleaning station 43, where the lines are removed, for example by wipingwith solvent.

If the markings pass the quality test, the substrate continues to afinalising station 44. In the case of a UV curable ink, the finalisingstation comprises a UV light source with an exposure time and intensitysufficient to substantially fully cure the ink.

The embodiment of FIG. 4 describes the use of a photo curable ink.However, other reagents may be used to apply the markings. The criteriafor choice of reagents are that the application is reversible, withfinalising being a separate step and that the applied reagent before itis finalised is detectable to enable checking. Preferably, theappearance of the marking pre and post finalising remains the same, sothat the checking step gives a good indication of quality of thefinalised marking.

An alternative to a photo curable ink is photo initiator etching. Thiswould preferably entail the use of an opaque and stable photo initiatorreagent which can be applied to the scale substrate. It shouldpreferably be sufficiently opaque to be detectable by the visioninspection machine and sufficiently stable to not react with the scalesubstrate until activation in the finalising step. Once the markingshave been applied and checked, the markings may be finalised using alight source to activate the reagent. The process may require an extrastep of removing excess reagent once the marking has been etched.

This invention is particularly useful for markings to be applied to thescale substrate in situ. Incremental scales are used in a variety ofapplications, such as machine tools, pick and place machines, maskaligners, CMMs etc. Once the scale has been mounted in position, it maybe desirable to place a reference mark at a certain position particularto that machine, for example in a ‘home’ position in the centre of thescale. Furthermore, it may be desirable to place limit switches on thescale to mark the end of allowable motion of movable parts. Preferably,any reference mark or limit switch is robust, particularly in harshenvironment such as a machine tool. However applying markings in situhas the disadvantage that if the marking is incorrectly applied, theinstalled scale must be removed and replaced. Thus the present inventionis particularly suitable for applying a marking in situ as it enablesthe marking to be checked before it is finalised to create a robustmarking. If necessary, a wrongly applied marking can be removed andreapplied.

FIGS. 5( a), (b) and (c) illustrates a device 50 to aid an operator tocorrectly apply the marking. The device comprises an upper housing part52 and a lower housing part 54. A scale locator comprising a groove 68with parallel straight edges 70,72 is located on the bottom surface ofits lower part 54. The device has an applicator in the form of a bladeedge 62 of a blade 60 and a reagent storage region 64, 66 in the form offine grooves 66 on the blade 60 leading to the blade edge 62. The bladeis mounted on a side face of the upper part of the housing with itsblade edge 62 perpendicular to the edges 70,72 of the groove 68. Twopins 56, 58 extend from the lower part of the housing into two aperturesof the upper part of the housing, allowing motion up and down relativeto the lower part of the housing. The upper part of the housing isbiased away from the lower part by springs 59 located on the pinsbetween the upper and lower parts.

In use the device is placed on a portion of scale substrate, so that thescale substrate fits into the groove, with at least one edge of thegroove abutting the straight edge of the scale substrate. In thisposition, the blade is positioned above the scale substrate andperpendicular to the longitudinal, or measurement, axis of the scalesubstrate. The blade is not in contact with the scale substrate, due tothe upper part of the housing being biased away from the lower part. Toapply the reagent held in the reagent storage region to the scale, thehousing is moved along the scale substrate to the desired position andthen the upper part of the housing is depressed, bringing the blade edgeinto contact with the scale substrate. The upper part of the housing isthen released.

Typically the reagent storage region contains a sufficient amount ofreagent for one marking. The reagent storage region may be replenishedby placing the blade edge against a reagent supply, such as an ink pad,enabling the reagent to travel into the reagent storage region bycapillary action.

Once the marking has been applied to the scale substrate, it is checkedfor quality. This may be a visual check by an operator or alternativelythe marking may be checked by passing a readhead over the scale anddetermining whether the marking has been detected. If the marking passesthe quality test, it is finalised. The device may include a curingsource, such as UV source in the housing. Alternatively, the marking maybe finalised using a separate hand-held device, such as a UV pen.

In an alternative embodiment, the device could be mounted onto themounting for the readhead, the scale locator thus comprising mountingfixtures instead of or as well as the grooves. For example the devicemay be provided with a fixture for mounting to the bolt holes of themachine provided for mounting of the readhead. Alternatively, the devicecould be mounted to the readhead itself, enabling the readhead to beused to check the marking whilst the device is attached. In such anembodiment, the ink or other reagent may be applied remotely, forexample using air pressure generated by pushing a plunger to push theink from the reagent storage area to the blade edge or remote operationvia a tensile element to depress the upper housing part.

FIG. 6 shows an alternative device suitable for applying the marking. Anadhesive mask 80 is provided with an alignment feature 82 in the form ofan elongate line for aligning with the scale edge and an appropriatelysized slit 84 for the marking. The adhesive mask is stuck onto the scalesubstrate, aligning the elongate line with the scale edge and placingthe slit in the appropriate position for the marking.

The mask may comprise a laminate, for example a 3 ply laminate, whichhas sufficient thickness to be self supporting. The bottom layer isremoved to expose a low tack adhesive layer, enabling the remaining 2ply mask to be mounted to the scale; the remaining two layers stillproviding sufficient thickness to be self supporting. The top two layersare preferably transparent for ease of positioning the elongate line andslit. Once in place, the top layer is removed to leave a single layer ofmask in place, which has the desired thickness of the resulting marking.

Ink may then be applied to the slot and excess ink removed by passing ablade over the top surface to leave just the slot full of ink. This maybe inspected, and if acceptable, cured by means of, for example, a UVlight source. Peeling off the mask will leave the cured mark adhered tothe scale. If not cured, the mark may be wiped off, after mask removal,for example with IPA.

FIG. 7 shows an embodiment which uses a tampo printing technique toplace the marking. An ink block 86 is provided with a groove 88 which isfull of ink. The groove 88 is designed to hold the correct volume of inkfor one marking and is shaped to give correct shape and transfer of theink onto the scale substrate. The block may be loaded with ink usingtechniques such as inkjet delivery or an ink pad. The block is thenplaced onto the scale substrate, with the groove in the desired positionof the marking, to transfer the ink to the scale substrate. The blockmay be used in a device which gives correct alignment, for example inplace of the blade in FIG. 5. Once the ink has been transferred to thescale substrate, the marking can be checked and finalised, as inprevious embodiments.

FIG. 8 shows an alternative design of ink blade 90, suitable for usewith the device of FIG. 5. An ink blade body is provided with an inkreservoir 92 to hold sufficient ink for a single marking and inktransfer channels 94 between the reservoir and the blade edge 96. Thereservoir and channels are covered by a flexible cover 98, such as foil,which can be depressed to expel the ink.

FIG. 9 shows an alternative embodiment of the device for applying themark. The device is a body 100 with a base 102 having a scale locatorcomprising a groove for engaging with the scale substrate, in the sameway as FIG. 5. The body 100 holds an applicator 104, for example a bladeas in FIG. 5 or 8 or a block as in FIG. 7. A region of the body isresilient, for example due to reduced thickness, thus enabling theapplicator to be pushed against the scale substrate.

The thickness of the scale marking is dependent on the thickness of theapplicator, for example the blade edge in FIGS. 5, 8 and 9. If too muchink 110 gathers at the tip of the blade 112, as illustrated in FIG. 1OA, the marking may become wider than desired. FIG. 10B illustrates ascraper 114 for scraping excess ink from the blade. The scrapercomprises a housing 114 which holds an ink reservoir 116, for example anink pad. Two round edged scraper blades 118 are positioned above the inkreservoir, so that a blade inserted into the ink reservoir must bepushed through the space 120 between the scraper blades. On removal ofthe blade from the reservoir, excess ink is thus removed from the sidesof the blade by the scraper blades.

FIG. 10C illustrates a blade tip 112 after it has been removed from thescraper of FIG. 10B. Ink has been removed from the side of the blade tip112, leaving a bead of ink 122 at the tip. A similar result could alsobe achieved by, for instance, coating the sides of the blades 112 with asurface having a low coefficient of friction, such aspolytetrafluoroethylene.

It is also desirable to be able to apply a temporary reference mark to ascale substrate. For example, big machines will have large lengths ofscale and it may be desirable to work within a small working area on themachine. In this case, it would be useful to apply a temporary referencemark to the scale to define the small working area. This temporaryreference mark can be removed once the work is finished.

In a first embodiment of temporary reference mark, a solvent based inkis used to apply the marking. As a solvent based ink is used, no curingis necessary. An ink is chosen which is removable using a solvent, forexample acetone or xylene. Preferably, the marking can be cleaned with aseparate solvent, such as isopropyl alcohol without removing themarking. Thus a robust marking is produced which can be removed when nolonger required.

In a second embodiment, a marking is located on a transparent adhesivecarrier. This carrier may have a similar design to the sticker in FIG. 3c. The carrier is placed onto the scale substrate with the marking inthe appropriate position and held in place by the adhesive. This markingmay be used as a temporary reference mark, by choosing an adhesive whichenables the carrier to be applied and removed. Alternatively, it couldbe used as a permanent reference mark, by choosing an adhesive which ispermanent, for example the adhesive may be photo-initiated, e.g. byexposure to UV light.

In the above described embodiments, the encoder is an optical encoder,with the incremental scale being defined by a series of reflective andnon-reflective features detectable by the readhead. In the embodimentsdescribed, the reference mark is a non-reflective, i.e. light absorbent,feature that is a number of periods wider than an incremental feature.As will be understood, there are other ways of making an opticallydetectable reference mark, for instance by providing a light scatteringreference feature, a specular reference feature with the reflectingsurface that steers illumination light in a different direction to thatreflected from a specular substrate, or a refracting reference featurethat directs or shapes illumination light in a way that differs to thesurrounding material, e.g. a cylindrical lens on a plane glass surface.Furthermore, the scale could be a transmissive scale and comprise aseries of transmissive and opaque features.

In the above described embodiments, the scale's position features andthe reference mark are defined by features having optical propertiesthat are detectable by the readhead. However, it will be understood thatthis need not necessarily be the case and for instance the incrementalposition features and/or reference marks can be formed by featureshaving different properties. For instance, as is well known, featureshaving different magnetic, capacitive or inductive properties can beused to encode position information onto a scale. In these cases anappropriate magnetic, capacitive or inductive sensor arrangement will beprovided in the readhead.

Furthermore, it will be understood that the scale's position featuresand the reference mark need not be detectable via the same type ofproperty. For instance, the scale's position features could be definedby features having particular optical properties whereas the referencemark could be defined by a feature having a particular magnetic,capacitive or inductive property.

As will be understood, in the case of a magnetic encoder, the scale canbe passive or active. An example of passive scale could be a ferrousmaterial with regular slots. Active scale could be fabricated frommagnetised lamina stacked along the measuring direction or byselectively setting the polarisation of a series of sections of amagnetic strip. A reference marking on such magnetic scale could operateand be detectable by it altering the permeability of the space betweenthe scale and readhead. For example, the reference marking couldcomprise an ink loaded with ferrous particles (which could be magneticor ferromagnetic). Such an ink could be used to fill one or more slotsin passive magnetic scale to provide a measureable disruption to theperiodic permeability of the surface or bridge one or more polejunctions in active scale to give a measureable disruption to theperiodic flux pattern near the scale. The ink could be initiallyprovided on the scale in a fluid and hence provisional state and oncechecked could be cured in any of the manners described above in order tofinalise it.

In the case of capacitive scale, a reference mark could be one thatchanges the permittivity of the space between the readhead and scalewhich is detectable by a capacitive detector. An ink with significantlydifferent dielectric constant (or loaded to be so) compared to thesurroundings (usually free space) could be deposited on the measuringscale to provide a detectable disruption to the periodic capacitivesurface. As with the above described embodiments, the ink could beinitially provided on the scale in a fluid and hence provisional stateand once checked could be cured in any of the manners described above inorder to finalise it.

As will be understood, the invention is not limited to the applicationof marks on incremental scales but could also be used for instance withabsolute scales, and in particular for applying limit marks to anabsolute scale.

Furthermore, the invention is suitable for use with linear and rotaryscales (for instance ring scales), and for scales providing measurementinformation in one or more measuring dimensions. For example, theinvention is also suitable for use with scales providing measurementinformation in two dimensions.

1. An apparatus for applying a marking to a scale substrate, comprising:a scale locator for positioning the apparatus in a defined positionrelative to the scale substrate in one dimension; an applicator forlocating one or more markings on the scale substrate in a provisionalstate in which the one or more marking can be removed; a detector forchecking the markings; and a finalizer for finalizing the one or moremarkings which are acceptable so as to transform the one or moremarkings into a permanent state in which the one or more markings areless readily removable than in the provisional state.
 2. An apparatus asclaimed in claim 1, further comprising a remover for removing from thescale substrate one or more markings which are not acceptable.
 3. Anapparatus as claimed in claim 1, in which the applicator locates the oneor more marking by applying a marking to the scale substrate.
 4. Anapparatus as claimed in claim 3, in which the applicator applies themarking in a liquid state.
 5. An apparatus as claimed in claim 4, inwhich the liquid comprises an ink.
 6. An apparatus as claimed in claim4, in which the finalizer transforms the one or more markings into asolid state.
 7. An apparatus as claimed in claim 6, in which thefinalizer cures the one or more markings.
 8. An apparatus as claimed inclaim 3, in which the applicator applies the marking in a solid state.9. An apparatus as claimed in claim 8, in which the marking comprises anadhesive for securing the marking to the scale substrate and thefinalizer transforms the adhesive.
 10. An apparatus as claimed in claim1, in which the applicator locates the one or more marking bypositioning a marking carrier adjacent the scale substrate and thefinalizer finalizes the one or more markings by transferring the markingfrom the marking carrier onto the scale.
 11. An apparatus as claimed inclaim 1, in which the finalizer finalizes the one or more markings byexposing the one or more markings to an electromagnetic radiation(“EMR”) source.
 12. An apparatus as claimed in claim 11, in which theEMR source is an ultraviolet (“UV”) source.
 13. An apparatus as claimedin claim 1, in which the detector checks the markings by checking theposition of the marking.
 14. An apparatus as claimed in claim 1, inwhich the finalizer finalizes the one or more markings by exposing theone or more markings to a heat source.
 15. An apparatus as claimed inclaim 1, in which the finalizer finalizes the one or more markings byapplying pressure to the one or more markings.
 16. An apparatus asclaimed in claim 1, in which the one or more markings comprises areference mark.
 17. An apparatus as claimed in claim 1, furthercomprising a pinner for pinning the marking prior to finalization.
 18. Akit of parts comprising: a scale comprising a series of positionfeatures; and an apparatus for applying a reference mark onto the scale,the apparatus carrying marker matter which can be applied to a scalesubstrate in a provisional state in which the marker matter can beremoved and subsequently transformable into a permanent state in whichthe marker matter is less readily removable than in the provisionalstate on the scale substrate via a finalization procedure.
 19. A kit ofparts as claimed in claim 18, in which the apparatus comprises a storageregion for the marker matter and an applicator for transferring themarker matter from the storage region onto a scale substrate.
 20. A kitof parts as claimed in claim 19, in which the marker matter is a liquidand the applicator comprises at least one conduit for transferring themarker liquid from the storage region onto a scale substrate.
 21. A kitof parts as claimed in claim 18, in which the marker matter comprises anobject comprising an adhesive backing that is transformable into apermanent state on the scale substrate via a finalization procedure. 22.A kit of parts as claimed in claim 18, in which the marker matter is adecal and the apparatus comprises a decal carrier sheet.
 23. A kit ofparts as claimed in claim 22, in which the decal carrier sheet does notinterfere with the detection of markings on a scale substrate on whichthe decal carrier is placed.
 24. A kit of parts as claimed in claim 18,in which the apparatus comprises features for aiding alignment of themarking to the scale substrate.
 25. A kit of parts as claimed in claim24, in which the apparatus comprises a scale locator for positioning theapparatus in a defined position relative to the scale substrate in atleast one dimension.
 26. A kit of parts as claimed in claim 18, furthercomprising a tool for finalizing the marking.
 27. An apparatus forapplying a reference mark onto the scale, for use with a kit of parts asclaimed in claim 18.